Cellular communication systems are known and include a number of communication cells. The cells, each of which contain a base site transceiver (BST), are linked by a base cite or base station controller (BSC) which in turn is linked to a mobile switching center (MSC) and then to the public switch telephone network (PSTN) or other mobile switching centers. A communication unit or mobile station (MS) operating within the cellular system communicates with a base site transceiver which receives and processes the data from the communication unit to the base cite controller and processes and transmits data received from the base cite controller to the communication unit. The cells cover a defined geographical area and when the communication unit travels within the cell, communication takes place solely with one cell transceiver. However, if the communication unit moves out of this cell, the cellular system must provide for transferring or handing off the communication unit from the first or source cell to a target cell, i.e., the cell where the communication unit is moving into.
Where communication has been established between a communication unit and a source cell transceiver, and the communication unit moves from the source cell to a target cell, the communication link must be reestablished through the base site transceiver of the target cell. As an aid in identifying a target cell, the communication unit typically measures signal strengths or other characteristics of signals from the surrounding cell base site transceivers. The communication unit transmits these measurements and the identity of the associated base site transceiver to the source cell transceiver. The source cell transceiver, in turn, transfers the signal strength measurements to the base site controller and the mobile switching center for evaluation and hand-off decision making.
Hand-off processing within a cellular communication system has long posed a number of problems. Namely, the user of the communication unit should not experience undue disruption or loss of service when a hand-off is made. At the same time, the communication link between the source cell transceiver and the communication unit must be broken and communication reestablished between the communication unit and a target cell transceiver in a process generally referred to as hard hand-off.
During such hand-offs, a common disruption of service is known as audio hole. Audio hole occurs when data being communicated either from or to the communication unit or from or to the cell transceiver is lost, resulting in a noticeable break in otherwise continuous communication. In voice communication, for example, audio hole may appear as a momentary discontinuity of speech. Other disruptions may be caused due to a change in communication timing between the source cell transceiver and the target cell transceiver, or due to other processing features, such as echo canceling, which must be readapted after the hand-off is completed.
A second procedure, intended to reduce the disruptions associated with hand-off is commonly referred to as "third party conference" hand-off. In a cellular system incorporating a third party conference hand-off procedure, additional transcoder circuits are provided at the base site controller to accommodate hand-off processing. Such an arrangement is shown in FIGS. 1A and 1B. During hand-off, a second transcoder is brought on-line and begins receiving and processing data from a target cell transceiver. At the same time, a transcoder still processes data received from the source cell transceiver. The data from each transcoder is summed in a three-party conference (TPC) circuit and is communicated to the mobile switching center. When communication is fully established between the target transceiver and the communication unit, the hand-off is completed and communication with the source cell transceiver is discontinued. Further communication is conducted solely through the target transceiver.
While reducing some of the disruptions associated with hand-off processing, the third party conference hand-off technique suffers the distinct disadvantage of requiring additional transcoder circuits to accomplish this purpose. These additional transcoder circuits either have to be physically added to the cellular system, which increases the cost of the system, or the capacity of the system has to be reduced to ensure an adequate supply of transcoder circuits are available for use during hand-off processing. In either case, transcoder circuits are not efficiently utilized to the benefit of the cellular system operator.